Ceramic matrix composites (CMC) are crucially and widely used in aircraft, aerospace, weapons, ships, armor protection, high-speed brakes and other fields. Silicon carbide (SiC) matrix composites have a series of advantages, such as high temperature resistance, high strength, high modulus, low density, low coefficient of thermal expansion, and so on, becoming a new generation of strategic thermal structural materials.
A polymeric precursor for silicon carbide ceramic is a matrix resin for the preparation of silicon carbide fibers and silicon carbide matrix composites by the PIP method, and a key raw material for the production of silicon carbide-based high-temperature resistant adhesive, connective and composite ceramics and other fields. It can also be used to prepare a series of high-tech materials from zero-dimension to three-dimension, such as ultrafine ceramic powder, silicon carbide fiber and coating, silicon carbide ceramic, silicon-based advanced composite ceramic, high-temperature resistant resin, and so on.
Polycarbosilane (PCS) prepared by thermal-decomposition and rearrangement (about 450° C.-470° C.) poly(dimethylsilane) (PDMS) invented by Japanese Yajima has recently been used as the polymeric precursor for silicon carbide ceramic, which is the most mature technologies and the largest amount both at home and abroad, and also as the precursor for continuous silicon carbide fiber, which is the only one achieved for commercial application currently. Rearrangement under high pressure and atmospheric pressure are the main method for the preparation of polycarbosilane via rearrangement of poly (dimethylsilane) at high temperature both at home and abroad. The synthesis yield by high pressure method is higher, generally 45%-60%; the quality of the product is better, however, the equipment requirements by high pressure method are increased, and the equipment maintenance requirements are also enhanced, so it needs a relatively large investment and high costs. The synthesis time by atmospheric pressure method is longer and the synthesis yield is lower, generally 30%-42%, therefore, for the atmospheric pressure method, it is also to face high cost problems.
Shortening reaction time, improving synthetic yield and reducing reaction temperature are the current main research directions in the PCS preparation technology under atmospheric pressure. The study has been shown that, when polycarbosilane was prepared by rearrangement at high temperature and under atmospheric pressure, adding catalysts could shorten the synthesis time and improve the synthesis yield. For example, Japanese Y. Hasegawa and K. Okamura had reported that when adding 3-5 wt % of polyborosiloxane as a catalyst, the reaction temperature could be reduced to 350° C. After the reaction was carried out at 350° C. for 10 hours, the yield was up to 50% or more. However, the product had higher O content, lower Si—H bond content, higher Si—Si bond content (11-14%), low ceramic yield and poor storage stability, which could not be spun. When a boron-oxygen catalyst such as tributyl borate B(OBu)3 was added, the similar drawbacks existed. Adding a halide catalyst such as MCl3 (M=Al, Mn, Ti, V), the harmful chlorine ion could be introduced and the ceramic yield of the product could be lower. CN10258535A patent disclosed a method for catalytic rearranging using alumina, Al—Si oxide, Si—Ti oxide or Al—Si—Ti oxide with a high surface area as a catalyst, which resulting in the polycarbosilane synthesis yield of 56-72%, but the reaction temperature in the method was still as high as 460° C. South Korean Dong-Pyo Kim et al. reported that when aluminum silicate molecular sieves were used as a solid acid catalyst, it could significantly reduce the temperature and pressure in the high pressure method.
In summary, in the current methods for the preparation of polycarbosilane via thermal decomposition and rearrangement, whether under atmospheric pressure or high pressure process, it is difficult to achieve the goals simultaneously, such as short reaction time, high yield, lower reaction temperature, good product quality, thus significantly reducing the preparation cost.